Internal combustion reciprocating (and/or linear) piston engines operating with a compression ignition (CI) design compress the fuel and oxidizer to the point of auto-ignition rather than using a spark ignition as is common in gasoline engines. In the homogeneous charge compression ignition (HCCI) mode the fuel and oxidizer are well mixed prior to compression. In the stratified charge compression ignition (SCCI) mode the fuel is injected during the compression stroke. In compression ignited systems, the auto-ignition timing is inherently related to the chemical properties of the fuel charge and can also be controlled by the stratification of the charge. Both compression ignition modes can achieve higher energy conversion efficiencies at lower operating temperatures than spark ignition engine operations. HCCI can substantially reduce nitrogen oxide (NOx) emissions without a catalytic converter.
Currently, unburned hydrocarbon particulates and carbon monoxide (CO) emissions from CI systems require post-combustion treatment, but if improvements can continue to be made in these systems, they offer the potential to completely eliminate NOx and particulate emission after-combustion requirements. For instance, research has shown that the lower operating temperatures of reactivity controlled compression ignition (RCCI) systems can lead to substantial reductions in NOx emissions over wide load and speed ranges, even to levels such that after-treatment removal of NOx is no longer needed.
The chemical kinetic properties of ignition systems are frequently characterized by standardized testing methods. In the case of gasolines, these methods produce the octane numbers (ON) including the research octane number (RON) and motored octane number (MON) for the fuel, while the cetane number (CN) is used for characterizing diesel fuels. The engine operating characteristics utilized in standard ASTM test methods for determining these reference indicators are different and specific to each of the above rating numbers. In general, the properties of gasolines are configured to produce higher octane numbers, which indicate that the fuel is resistant to autoignition, while the properties of diesel fuels are configured to produce higher Cetane numbers, indicating an ability to readily ignite. It is well established empirically that the Octane and Cetane scales are inversely proportional to one another. As a result, one approach to improving control of RCCI ignition and burn rates is through formation of a combustible charge for an engine cylinder using a hybrid fuel including a first fuel having a high octane number combined with a second fuel having a high cetane as a means of varying the propensity of the fuel charge to autoignite.
Another more traditional means of varying the autoignition properties of a fuel is through the use of chemical octane or cetane improvers. A historical octane improver is tetraethyl lead, which over time has been removed from consideration due to its lead content. Another is methylcyclopentadienyl manganese tricarbonyl(MMT or MCMT). Common cetane improvers include alkyl nitrates (principally 2-ethylhexyl nitrate, 2-EHN) and di-tert-butyl peroxide (DTBP). Varying the additive level or its effectiveness in modifying the CN or ON properties can be applied to vary the octane or cetane character of a fuel charge, including a hybrid fuel charge.
While the above principles have been supported by experiment, controlling autoignition and burn rate for liquid-fueled RCCI cycles by varying fuel properties on the fly (i.e., continuously during engine performance) according to these known methods would require complex storage of multiple materials on the vehicle (two or more fuel types, fuel additives) and/or the use of the expensive additives as well as a complicated control system to constantly monitor and modify fuel ratios and/or additive amounts.
What are needed in the art are methods and products that can reform fuel so as to better tailor engine combustion and emissions properties. In particular, what are needed are methods and systems that can allow for variation of a single fuel supply to dynamically achieve a range of autoignition and burn rate chemical properties.